Compound semiconductor devices using GaN or GaN based compound semiconductor are under active development. GaN has a wide band gap of 3.4 V allowing a high voltage operation. Various types of semiconductor devices can be manufactured by forming a hetero junction of GaN based compound semiconductor. Metal organic chemical vapor deposition (MOCVD) is mainly used as a crystal growth method for the GaN based compound semiconductor. A method called hydride vapor phase epitaxy (H-VPE) has been under research recently, which method grows nitride semiconductor by reacting HCl and group III metal to form metal chloride which is further reacted with ammonia or the like to form nitride semiconductor.
A semiconductor light emitting device using GaN based compound semiconductor can emit blue or ultraviolet light, and can form a white light source by using phosphors. Various light emitting devices are manufactured by growing GaN based compound semiconductor crystal on a sapphire substrate or on a SiC substrate.
GaN has a high breakdown voltage, and is expected for applications in a field requiring a high voltage operation and a high speed operation such as high electron mobility transistors (HEMT) used in a mobile phone base station. In addition, GaN is also expected for inverter/converter related systems such as power supply, car control unit, power plant. Various types of GaN-HEMT have been reported having a GaN layer as an electron transfer or channel layer among GaN/AlGaN crystal layers grown on a substrate such as sapphire, SiC, GaN and Si. A breakdown voltage over 300 V in a current-off state is presently reported. The best output characteristics are now obtained in GaN-HEMT using a SiC substrate. A high thermal conductivity of SiC is considered to contribute to this performance. In manufacturing a high speed operation GaN device, a semi-insulating SiC substrate is used to suppress a parasitic capacitance. However, a price of a semi-insulating single crystal SiC substrate is high, which may possibly hinder the spread of GaN based semiconductor devices. A conductive SiC substrate is available at a lower price than a semi-insulating SiC substrate. However, when a semiconductor device is formed on a conductive SiC substrate, parasitic capacitance increases.
JP-A-2002-359255 proposes to grow, as an underlying layer, an AlN layer to a thickness of 2 μm on a conductive SiC substrate having resistivity smaller than 1×105 Ωcm by metal organic chemical vapor deposition (MOCVD) and form device constituent layers on the underlying layer.
JP-A-2003-309071 proposes to grow an AlGaN underlying layer above a crystal substrate such as sapphire and SiC via an AlN low temperature growth buffer layer by MOCVD and form an AlGaN layer on the underlying layer.
International Publication No. 2004-524690 proposes a hybrid growth system capable of growth in an MOCVD mode and growth in an H-VPE mode in the same chamber and capable of growth in both the modes.
JP-A-2005-252248 proposes to suppress a growth chamber temperature not higher than 750° C. not forming deposition and maintain a substrate at a temperature of 900° C. to 1700° C. by high frequency heating, during H-VPE growth of AlN crystal on a sapphire substrate or on a Si substrate.
MOCVD is difficult to make the growth speed high, and is not suitable for growth of a thick GaN based semiconductor layer. Although it is known that H-VPE has the merit of a high growth speed, H-VPE is a crystal growth method under development, and there are some future issues of findings of problematic points, their solutions and the like.